1. Field of the Invention
The present invention relates to the field of power electronics, and particularly to a switching regulator and a control circuit and a control method therefor.
2. Description of the Related Art
A switching regulator typically comprises a power stage and a control circuit. The control circuit can consider variations in an input voltage, internal parameters and external loads, and may regulate the ratio of on/off time of the switch system in the power stage, so that an output voltage or an output current of the switching power supply can be maintained as substantially constant. Therefore, design of a particular control circuitry and approach is very important to the overall performance of the switching regulator. Thus, adopting different detection signals and/or control circuits may result in different control effects on performance.
The control patterns of a switching regulator comprise constant frequency control and variable frequency control. The constant frequency control (i.e. Pulse Width Modulation, PWM) is a control method in which the switching cycle is maintained as constant, and the voltage regulation is achieved by adjusting on time of a switch in a switching cycle.
The variable frequency control (i.e. Pulse Frequency Modulation, PFM) comprises various control methods such as constant on time control, constant off time control, current hysteresis control, etc. In constant on time control, the on time of a power switch is maintained as constant, and the duty cycle is adjusted by changing the off time of the power switch. In constant off time control, the off time of the power switch is maintained as constant, and the duty cycle is adjusted by changing the on time of the power switch.
In constant on time control, when load of the switching regulator varies, for the purpose of power loss reduction, the prior art typically reduces the on/off frequency of the power switch in the power stage by omitting or shielding pulses (i.e. reducing the frequency of switching control signal), so that the control circuit operates in an intermittent manner.
However, when the frequency of switching control signal drops into audio range (20 Hz-20 KHz), the on/off of the switch may lead to audio noise.